This invention relates to vehicle braking systems of the kind incorporating two circuits serving driven and non-driven axles respectively and in which both air-actuated brake units (service brakes) and spring brake units (parking or secondary brakes) are provided on both axles. In such systems it is required that the spring brake units be operable by a hand control valve separate from the service brake pedal. It is also desirable that in the event of a failure of the service braking on either axle secondary braking should be brought into operation on that axle by the service foot control. It is further desirable for reasons of vehicle stability during braking, and particularly when the vehicle is less than fully laden, to reduce the degree of braking on the rear (driven) wheels in relation to that on the front (non-driven) wheels, and it is an object of the present invention to provide an improved braking system which meets these requirements.
According to the invention, a vehicle braking system of the kind referred to includes a hand control valve for venting the spring brake units for braking purposes, driver operated air valve means for effecting service actuation of the air-actuated units, and relay valve means operable on failure of the service braking in either circuit and responsive to the service pressure in the other circuit to provide a graduated actuation of the spring brake units in the circuit where service braking has failed, the spring brake units on the driven axle, and/or the relay valve means controlling operation of such units, being so constructed as to attenuate the force applied by the spring brake units on the driven axle.